Methods and devices for fluid enhanced microwave ablation therapy

ABSTRACT

Devices and methods for fluid enhanced ablation therapy using microwave antennas are described herein that provide improved heat transfer into a target volume of tissue and improved antenna cooling compared to prior art designs. In one embodiment, fluid can be introduced into a target volume of tissue along with the delivery of therapeutic energy from a microwave antenna positioned within the target volume of tissue. The fluid can become heated by cooling the microwave antenna and, if additional heating of the fluid is desired, a heating element disposed within the microwave antenna can supplementally heat the fluid prior to introduction into the target volume of tissue.

FIELD

The present invention relates generally to fluid enhanced ablationtherapy, and more particularly, to devices and methods for conductingfluid enhanced ablation therapy using microwave antennas.

BACKGROUND

Ablation therapy uses heat to kill undesirable tissue. Energy fromvarious sources, often radiofrequency (RF) electrical energy, but alsomicrowave, ultrasound, or laser energy, is used to heat tissue to atherapeutic temperature and thereby kill the tissue. This therapy can beused to treat various small tumors, including malignant tumors such asliver cancer or benign tumors such as uterine fibroids, by overheatingand killing the tumorous tissue. It can also be used to ablatearrhythmogenic heart tissue that causes arrhythmias, such as atrialflutter. While ablation therapy is often used to treat small amounts oftissue such as those mentioned above, conducting the therapy for largertumors, or for other conditions that require treating large volumes oftissue, remains difficult.

One prior art solution for ablating larger volumes of tissue has beenthe use of microwave energy in place of RF electrical energy. Microwaveenergy can be better suited for use in large treatment volumes becauseits energy can penetrate farther into tissue than RF energy. Forexample, microwave energy at 915 MHz can penetrate approximately 4 cm(in theory) into tissue, as compared to RF energy that is typicallydissipated beyond about 1 cm from the electrodes that deliver theenergy. Therefore, microwave energy can directly heat a greater volumeof tissue than RF energy.

Microwave energy can have additional advantages over other energysources as well. For example, microwaves propagate through all types oftissues and non-metallic materials, including charred and desiccatedtissues that can be created during the ablative process. In contrast,when tissue becomes charred or desiccated by RF ablation the impedanceof the tissue rises dramatically, making it difficult to pass furthercurrent through the tissue and effectively terminating the therapy.Still further, microwaves can deliver greater levels of direct heatingenergy as compared to other ablation energies, which can be advantageouswhen ablation is conducted in organs with high blood perfusion or nearsources of blood flow, such as veins or arteries, that can draw heataway from a target tissue volume.

However, microwave energy does have drawbacks as well. For example,while the depth of treatment may extend farther into tissue than with RFenergy, it is still limited. As mentioned above, in theory the field istypically dissipated at about 4 cm from the microwave antenna. Inpractice, the cylindrical configuration of microwave antenna exacerbatethe radial spreading of that energy into tissue being treated, resultingin dissipation within 2 cm of the cylindrical antenna. Treating largervolumes of tissue therefore requires repositioning the antenna or usingmulti-antenna arrays. Further, the dynamics of the microwave field canbecome complex when multi-antenna arrays are used. As a result, therapyprocedure times and costs can be significantly increased due to eitherrepositioning a single probe several times or setting up a multi-antennaarray.

In addition, the microwave energy deposition field is static and isdefined by the electromagnetic properties of the surrounding tissues andthe geometry of the antenna itself. Within the heating field there arevolumes of tissue that are heated more or less than others (e.g.,similar to reheating food in a microwave oven). This can lead toundesirable therapies, as some tissue can be heated to a dangerous levelby a strong microwave field (e.g., becoming superheated and explosivelyconverting to steam), while other tissue can be heated to asub-therapeutic temperature by a weaker microwave field. Thermal energydoes flow from the volumes of tissue that are heated by strong microwavefields to those heated by weaker microwave fields by thermal conduction,but this is a slow and inefficient process in tissue, and does notaddress the safety concern created by superheating portions of a targetvolume of tissue.

Moreover, microwave energy can overheat the antenna used for therapyapplication and its associated cabling used to transfer power to theantenna from a generator. This internal heating must be countered bylimiting energy transmission or by actively cooling the components ofthe microwave ablation system to prevent undesired thermal damage totissues in contact with the antenna or the cabling extending to theantenna. Prior art methods for actively cooling microwave antennasinclude circulating a fluid or cryogenic gas along the length of thecable and even through the antenna itself, but these closed-loop systemscan result in large diameter devices, e.g., due to the need for deliveryand return lumens.

Accordingly, there is a need for improved devices and methods forconducting ablation using microwave antennas. In particular, there is aneed for microwave ablation devices and methods that can delivertherapeutic doses of thermal energy to large volumes of tissue andaddress cooling issues commonly encountered with microwave antennas.

SUMMARY

The present invention generally provides devices and methods for usingmicrowave antennas in combination with fluid enhanced ablation therapy.For example, the methods described herein generally include placing amicrowave antenna within a target volume of tissue and simultaneouslydelivering microwave electrical energy and a fluid into the tissue. Thefluid introduced into the tissue can aid in distributing the heat fromthe microwave field throughout the tissue volume via convection.Furthermore, in some embodiments, the fluid itself can be heated to atherapeutic level prior to being introduced into the tissue.Accordingly, the fluid can act as a therapeutic agent and the energydelivered through the microwave antenna can replenish heat lost from thefluid into the target tissue volume.

In addition to utilizing the fluid as a therapeutic agent to ablatelarger volumes of tissue, the devices and methods described herein canalso utilize the fluid to aid in cooling the microwave antenna orcabling extending between a power generator and the microwave antenna.Indeed, in some embodiments, excess heat from the microwave antennaand/or its associated cabling can be used to at least partially heat thefluid to a therapeutic temperature before introducing the fluid into thetarget volume of tissue, thereby putting traditionally wasted heat touse in the therapy. Still further, in embodiments in which a cryogenicgas is utilized to cool the antenna and/or associated cabling, a heatingelement disposed within a fluid passageway can be used in combinationwith the cooling action of the cryogenic gas to deliver fluid at anytemperature (e.g., any temperature above freezing) into the targetvolume of tissue.

In one aspect, a microwave ablation device is provided that includes anelongate body having an inner conducting element surrounded by adielectric insulator, an outer conducting element coaxially disposedaround the dielectric insulator and defining an outer wall, and at leastone fluid channel extending through the elongate body parallel to alongitudinal axis of the microwave antenna. The at least one fluidchannel can include at least one opening to allow fluid to flow intotissue surrounding the elongate body, and the inner and outer conductingelements can be configured to deliver microwave energy to tissuesurrounding the elongate body.

The devices and methods described herein can include a variety ofadditional features or modifications, all of which are considered withinthe scope of the present invention. For example, in some embodiments themicrowave antenna can have a cylindrical shape, and the inner conductingelement, dielectric insulator, and outer conducting element can becoaxially aligned. In other embodiments, the device can further includea shield disposed around and coaxially aligned with the outer conductingelement.

In other embodiments, the at least one opening can be positioned at alocation proximal to a distal end of the inner conductive element. Theat least one opening can, in some embodiments, include a plurality ofopenings. The plurality of openings can be spaced circumferentiallyaround a longitudinal axis of the at least one fluid channel, and/orthey can be spaced along a longitudinal axis of the at least one fluidchannel.

The at least one fluid channel can, in some embodiments, extend throughthe dielectric insulator between the inner conducting element and theouter conducting element, and the at least one opening can extendthrough the outer conducting element. Furthermore, in certainembodiments, the at least one fluid channel can include a heatingelement disposed therein. The heating element can be positioned proximalto the at least one opening and configured to heat fluid flowing throughthe channel.

In another aspect, an ablation device is provided that includes at leastone elongate body having proximal and distal ends and at least onemicrowave antenna extending from a distal end of the at least oneelongate body. The at least one microwave antenna can have an arcedshape that defines a plane, and the device can further include a fluidchannel extending through one of the body(s) and configured to deliverfluid into tissue from a center point of the arc defined by themicrowave antenna. The fluid channel can also include a heating elementdisposed therein and configured to heat fluid flowing through thechannel.

In some embodiments, the device includes first and second elongatebodies and first and second microwave antennas, with the first microwaveantenna extending from the distal end of the first elongate body and thesecond microwave antenna extending from the distal end of the secondelongate body. The planes of the arc shaped antennas can be angularlyoffset from one another such that the first and second microwaveantennas generally define a sphere.

In another aspect, an ablation device is provided that includes amicrowave antenna having a hollow outer conducting element, a coaxialinner conducting element extending through the outer conducting element,a fluid channel positioned between the outer and inner conductingelements, an inner lumen extending through the inner conducting element,and at least one outlet port formed in the outer conducting element andin fluid communication with the fluid channel. The at least one outletport can be configured to deliver fluid flowing through the fluidchannel into tissue surrounding the microwave antenna. The inner lumencan be configured to receive a cryogenic gas source. The fluid channelcan also include at least one heating element disposed therein andconfigured to heat fluid flowing through the channel.

In still another aspect, a method for ablating tissue is provided thatincludes delivering therapeutic energy through a microwave antennaconfigured to be positioned within a volume of tissue, andsimultaneously delivering heated fluid into the volume of tissue throughat least one outlet port formed in the microwave antenna, wherein thefluid is heated by contact with the microwave antenna.

In some embodiments, heating the fluid by contact with the microwaveantenna can have the additional benefit of cooling the antenna toprevent overheating. As a result, heat that is traditionally wasted canbe utilized in the therapy. If the fluid is not heated sufficiently bycontacting the microwave antenna, the method can further include raisingthe temperature of the heated fluid prior to introduction into thevolume of tissue using at least one heating element disposed within themicrowave antenna. In certain exemplary embodiments, the heated fluidhas a heat capacity that is equal to or greater than 2 J/ml-° C. (about½ the heat capacity of tissue).

In another aspect, a method for ablating tissue is provided thatincludes delivering therapeutic energy into a volume of tissue using amicrowave antenna, and cooling the microwave antenna by delivering acryogenic gas through at least one gas channel formed in the microwaveantenna. The method also includes delivering a fluid into the volume oftissue through at least one fluid channel formed in the microwaveantenna, and controlling the temperature of the fluid delivered into thevolume of tissue using the cooling of the cryogenic gas and a heatingelement disposed within the at least one fluid channel. By balancing thecooling effects of the cryogenic gas with the output of the heatingelement, fluid of almost any desired temperature can be introduced intothe volume of tissue. In certain embodiments, for example, thetemperature of the fluid delivered into the volume of tissue can becontrolled to be between a freezing temperature of the fluid and atemperature of the volume of tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

The aspects and embodiments of the invention described above will bemore fully understood from the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional diagram of a prior art microwave antenna;

FIG. 2 is a cross-sectional diagram of one embodiment of a microwaveantenna;

FIG. 3 is a graph showing microwave field strength and tissuetemperature when saline flow is not present;

FIG. 4 is a graph showing microwave field strength and tissuetemperature when saline flow is present;

FIG. 5 is a diagram of one embodiment of a fluid enhanced ablationtherapy system that includes a microwave antenna;

FIG. 6 is a diagram of an alternative embodiment of a microwave antenna;

FIG. 7 is a diagram of another embodiment of a microwave antenna;

FIG. 8 is a top view of the microwave antenna of FIG. 7;

FIG. 9 is a cross-sectional diagram of an alternative embodiment of amicrowave antenna;

FIG. 10 is a cross-sectional diagram of yet another embodiment of amicrowave antenna;

FIG. 11 is a cross-sectional diagram of one embodiment of a microwaveantenna assembly;

FIG. 12 is a cross-sectional diagram of the microwave antenna assemblyof FIG. 11 at location A-A; and

FIG. 13 is a cross-sectional diagram of the microwave antenna assemblyof FIG. 11 at location B-B.

DETAILED DESCRIPTION

Certain exemplary embodiments will now be described to provide anoverall understanding of the principles of the devices and methodsdisclosed herein. One or more examples of these embodiments areillustrated in the accompanying drawings. Those skilled in the art willunderstand that the devices and methods specifically described hereinand illustrated in the accompanying drawings are non-limiting exemplaryembodiments and that the scope of the present invention is definedsolely by the claims. The features illustrated or described inconnection with one exemplary embodiment may be combined with thefeatures of other embodiments. Such modifications and variations areintended to be included within the scope of the present invention.

The present invention is generally directed to devices and methods forfluid enhanced ablation therapy using microwave antennas. As mentionedabove, the devices and methods described herein generally includeplacing a microwave antenna within a target volume of tissue andsimultaneously delivering microwave electrical energy and a fluid intothe tissue. The fluid introduced into the tissue can aid in distributingthe heat from the microwave field throughout the tissue volume viaconvection. Furthermore, in some embodiments, the fluid itself can beheated to a therapeutic level prior to being introduced into the tissue.Accordingly, the fluid can act as a therapeutic agent and the energydelivered through the microwave antenna can replenish heat lost from thefluid into the target tissue volume. The introduction of fluid into thevolume of tissue in combination with the microwave field can effectivelydestroy tissue far beyond the approximately 2 cm practical limit ofconventional microwave antennas, and even well beyond the 4 cmtheoretical limit of conventional microwave antennas mentioned above.Further, the devices and methods described herein can allow for treatinglarge volumes of tissue with a single probe in a single position,thereby reducing therapy time and cost. Of course, the devices andmethods described herein can also be applied to microwave antennaarchitectures that utilize multiple probes.

Moreover, the devices and methods described herein can provide enhancedcooling to a microwave antenna or cabling extending between a powergenerator and the microwave antenna. Further, the excess heat drawn fromthe microwave antenna can raise the temperature of the fluid prior toits introduction into the target volume of tissue, thereby putting heatthat is typically wasted to use in the therapy. The heat from themicrowave antenna alone can be used to heat the fluid, or the fluid canbe supplementally heated by a heating element prior to being introducedinto the target volume of tissue. Still further, in embodiments in whicha cryogenic gas is used to cool the microwave antenna and/or itscabling, a balance of the cooling effects of the cryogenic gas and theoutput power of a heating element can be used to provide fluid at anydesired temperature—even temperatures below room or body temperature.

FIG. 1 illustrates one embodiment of a prior art microwave antenna 100and its associated microwave field shape and strength at variousdistances from the antenna. The antenna 100 is a triaxial architecturein a 17 gauge probe, as described by Brace, CL Crit Rev Biomed Eng.2010; 38(1): 65-78. The triaxial antenna 100 is a generally cylindricalantenna having a central inner conductor 102, such as an activeelectrode, extending along a longitudinal axis 103 thereof. The innerconductor 102 can be surrounded by a dielectric insulator 104 that iscoaxially aligned with, and extends over an outer surface of, the innerconductor. Extending over an outer surface of the dielectric insulator104 can be an outer conductor 106, such as a return electrode. Inaddition, a shield 108 can extend over an outer surface of the outerconductor 106.

The various components of the microwave antenna 100 can extend fordifferent lengths along the longitudinal axis 103 of the antenna. Forexample, the inner conductor 102 and the dielectric insulator 104 canextend along the entire length of the antenna 100, and the innerconductor 102 can be exposed at a distal end 110 thereof. The outerconductor 106 can extend to a first endpoint 112 that is proximal to thedistal end 110, and the shield 108 can extend to a second endpoint 114that is proximal to the first endpoint 112. This arrangement leavesspecific sidewall portions of the inner and outer conductors 104, 106exposed and results in a tear-drop shaped microwave field formed aroundthe antenna 100, as shown by field-strength lines 116, 118, 120.

As can be seen in the figure, the microwave field strength is strongest(e.g., 800 V/m) along the field strength line 116 that is closest to theantenna 100. The field strength dissipates with distance from theantenna 100, as shown by field strength lines 118 (e.g., 600 V/m) and120 (e.g., 200 V/m). In particular, the field strength can be greatestwithin about 4 mm of the antenna 100, and can fall to negligible levelsby about 1 cm from the antenna.

Thermal conduction can serve to treat volumes of tissue extendingfarther than 1 cm from the antenna 100, as heat can flow from the tissueheated directly by the microwave field into tissue that is unheated dueto the thermal gradients that result from the non-uniform field. Asmentioned above, however, it is accepted that the treatment field from asingle antenna 100 is limited to a distance of about 4 cm, and morepractically about 2 cm, from the antenna. Treating larger volumes oftissue can require the use of additional microwave antennas, or thesubsequent repositioning of the antenna 100 in various portions of thelarger volume to be treated. Using additional antennas can bechallenging and often adds significant time to the therapy because thedesired relative placements of the multiple antennae can be difficult toachieve and because the resulting microwave field can be difficult tomonitor.

The devices and methods of the present invention can utilize fluidenhanced ablation therapy to address these challenges. Fluid enhancedablation therapy, as mentioned above, is defined by passing a fluid intotissue while delivering therapeutic energy from an ablation element. Thedelivery of therapeutic energy into tissue causes hyperthermia in thetissue, ultimately resulting in necrosis. A variety of fluids can beused to conduct the therapy. In some embodiments, a sterile normalsaline solution (defined as a salt-containing solution) can be utilized.However, other liquids may be used, including sterile water, Ringer'ssolution, or concentrated saline solution. A fluid can be selected toprovide the desired therapeutic and physical properties when applied tothe target tissue and a sterile fluid is recommended to guard againstinfection of the tissue. In an exemplary embodiment, the fluid has aheat capacity that is substantially the same as or greater than the heatcapacity of tissue such that, as the fluid is delivery to the targettissue, the fluid can retain its temperature as heat is exchanged withthe tissue. For example, the fluid can have a heat capacity of greaterthan about 4 J/ml-° C., and more preferably greater than about 2 J/ml-°C. (which is about ½ the heat capacity of tissue).

One example of fluid enhanced ablation therapy is the SERF™ ablationtechnique (Saline Enhanced Radio Frequency™ ablation) described in U.S.Pat. No. 6,328,735, which is hereby incorporated by reference in itsentirety. The SERF ablation technique delivers fluid heated to atherapeutic temperature into tissue along with RF energy. Deliveringheated fluid in combination with microwave power can enhance theablation treatment because the fluid flow through the extracellularspace of the treatment tissue can increase the heat transfer through thetissue significantly. The flowing heated fluid can convect thermalenergy from the antenna farther into the target tissue. In addition, thefact that the fluid is heated to a therapeutic temperature increases theamount of energy that can be imparted into the tissue.

An advantage of fluid enhanced ablation therapy is that the use offlowing saline can avoid overheating the tissue located adjacent to themicrowave antenna because the heat cannot be efficiently transportedaway from the antenna. During fluid enhanced ablation therapy, thetherapeutically heated fluid can convect heat deeper into the targettissue, thereby reducing tissue charring near to the antenna. Further,because the fluid is heated to a therapeutic level, it does not act as aheat sink that draws down the temperature of the surrounding tissue.Therefore, the total volume of tissue that can be heated to therapeutictemperatures is increased. For example, experimental testing using RFelectrical energy has demonstrated that a volume of tissue having adiameter of approximately 8 cm (i.e., a spherical volume ofapproximately 156 cm³) can be treated in 5 minutes using fluid enhancedablation therapy techniques.

In addition, fluid enhanced ablation therapy devices have a greaternumber of parameters that can be varied to adjust the shape of thetreatment profile according to the tissue being treated. For example,operators or control systems can modify parameters such as salinetemperature (e.g., from about 40° C. to about 80° C.), saline flow rate(e.g., from about 0 ml/min to about 20 ml/min), output power (e.g., fromabout 0 W to about 200 W), and duration of treatment (e.g., from about 0min to about 10 min) to adjust the temperature profile within the targetvolume of tissue.

FIG. 2 illustrates one embodiment of a microwave antenna 200 that can beused in fluid enhanced ablation therapy. The antenna 200 shares thetriaxial architecture of the antenna 100 discussed above, including aninner conductor 202, dielectric insulator 204, outer conductor 206, andshield 208 extending along a length thereof. The antenna can have avariety of sizes according to its intended use (e.g., laparoscopictreatment of the liver, catheter-based treatment of the heart, etc.),the size of the target tissue volume, etc. In one embodiment, themicrowave antenna can be mounted on a 17 gauge probe having an outerdiameter of approximately 1.5 mm and a length of about 20 cm.

To facilitate its use in fluid enhanced ablation therapy techniques, theantenna 200 can include at least one fluid channel formed therein todeliver fluid into tissue surrounding the antenna. In the illustratedembodiment, the antenna 200 includes a single tubular or annular fluidchannel 210 that extends along a longitudinal axis 203 of the antenna200. As shown, the fluid channel 210 can be formed in the dielectricinsulator 204 and coaxially disposed around the inner conductor 202,with the dielectric insulator 204 separating the inner conductor 202from the fluid channel 210. In other embodiments, the antenna 200 caninclude two separate channels that extend along opposite sides of thelongitudinal axis 203 of the antenna 200, or any number of channelsdistributed around the circumference of the antenna 200. In addition,while the illustrated fluid channel 210 is formed through a portion ofthe dielectric insulator 204, in some embodiments the at least one fluidchannel can be formed at another location (e.g., between the outerconductor 206 and shield 208, or on an outer surface of the antenna200).

The at least one fluid channel can include at least one opening 212 toallow fluid to flow from within the channel into the tissue surroundingthe antenna 200. The at least one opening 212 can extend through theouter edges of the antenna 200 to provide fluid communication betweenthe at least one fluid channel and the tissue surrounding the antenna200. The at least one opening 212 can be formed at any point along thelength of the antenna 200, and the at least one fluid channel cansimilarly extend to any point along the length of the antenna 200. Insome embodiments, for example, the at least one opening 212 can includea plurality of openings formed along a distal portion of the fluidchannel 210 that terminates near the mid-point of the antenna 200,thereby introducing saline near a center of the microwave energydeposition pattern shown in FIG. 1. The plurality of openings can beformed in a variety of patterns, including, for example, in a patternextending radially around the outer circumference of the antenna 200 orextending in symmetrically opposed lines along the longitudinal axis 203of the antenna 200.

The at least one opening 212 can be formed in a variety of sizes,numbers, and pattern configurations. In addition, the at least oneopening can be configured to direct fluid in a variety of directionswith respect to the antenna 200. These can include the normalorientation (i.e., perpendicular to an antenna outer surface), as wellas orientations directed proximally and distally along the longitudinalaxis 203 of the antenna 200, including various orientations that developa circular or spiral flow of liquid around the antenna. Still further,in some embodiments, the at least one fluid channel can extend to adistal end of the antenna 200 and the at least one opening 212 can belocated on a distal surface of the antenna. A number of manufacturingmethods are available to create the at least one opening, including, inone embodiment, creating one or more openings having a diameter of about0.4 mm along a distal portion of fluid channel 210 using laser drilling.

In addition, the at least one fluid channel 210 can include a heatingelement disposed therein and configured to heat fluid flowingtherethrough. Exemplary heating elements include, for example,assemblies that are configured to pass electrical energy through fluidwithin the channel between two or more electrodes. Exemplary designs forheating elements suitable for use with the microwave antenna 200 andother similar devices are described in U.S. Pat. Pub. No. 2012/0265190,which is hereby incorporated by reference in its entirety.

One skilled in the art will appreciate that the at least one fluidchannel 210 and the at least one opening 212 can also be included in anumber of other microwave antenna architectures in addition to thetriaxial antenna 200. These can include, for example, slot, monopole,dipole, and choked microwave antenna architectures known in the art.

In use, fluid can be delivered into the tissue surrounding the antennasimultaneously with microwave energy from the antenna 200. The at leastone opening 212 can be positioned such that the fluid is introduced intothe tissue surrounding the antenna near the center of maximum heatingand can flow through the extracellular space of the tissue to deliver atherapeutic dose of thermal energy. The fluid can carry significantlymore energy deeper into the tissue because it carries the heat viaconvection.

By way of example, heat flowing in a given direction due to conductionis proportional to the thermal conductivity of the material k times themaximum temperature T_(max) divided by the length scale a. Bycomparison, the heat carried by a flow of fluid Q in the same directionis proportional to the heat capacity of the fluid (e.g., saline)p_(f)c_(f) times the maximum temperature T_(max) times the flow velocityof the fluid Q/4πa². Therefore, the ratio of the amount of energycarried by convection to that carried by conduction can be expressed bythe following equation:

$\begin{matrix}{\frac{Convection}{Conduction} \propto \frac{\rho_{f}c_{f}Q}{4\pi\;{ka}}} & (1)\end{matrix}$This proportion calculates to about 11 for typical values of tissuethermal properties and for a flow rate of saline of about 10 ml/min anda microwave field that peaks at about 1 cm from the antenna. Thus, heattransfer into tissue is improved by an order of magnitude when saline isinjected into tissue along with microwave energy.

FIG. 3 illustrates an exemplary microwave power deposition profile 302and tissue temperature profile 304 that can result from the use of themicrowave antenna 200 without any fluid injection (i.e., similar tousing the prior art antenna 100). While conduction does moderate thepeak temperature from what would occur if the tissue were perfectlyinsulating (i.e., zero thermal conduction), the peak temperature remainsnear the location of peak power deposition, i.e., close to the antenna200. Furthermore, both the power deposition level and tissue temperaturefall off quickly with distance from the antenna.

In contrast, FIG. 4 illustrates an exemplary power deposition profile402 and tissue temperature profile 404 that can result when the antenna200 is used in combination with a 10 ml/min saline flow rate. As thefigure shows, the temperature profile 404 is greatly modified. Inparticular, the peak temperature is reduced and the temperature profilebecomes more uniform, resulting in a more uniform thermal dose beingdelivered across the treatment zone. Furthermore, the therapeutictemperatures (e.g., temperatures sufficiently high to destroy tissue)extend deeper into the tissue, resulting in more tissue being treatedusing a single antenna.

FIG. 5 illustrates one embodiment of a fluid enhanced ablation therapysystem 500 that includes a microwave antenna 502 similar to the antenna200 described above. The antenna 502 can have a variety of sizesaccording to the geometry of the target tissue, intended method forintroduction into the target tissue, intended power output, etc. Theantenna 502 can be configured for insertion into a target volume oftissue in a variety of manners, including, for example, laparoscopicpercutaneous introduction or catheterization via a patient's circulatorysystem. In one embodiment, for example, the antenna 502 can be a probebetween about 16- and 18-gauge (i.e., an outer diameter of about 1.27 mmto about 1.65 mm), and it can have a length that is approximately 20 cm.The antenna 502 can include a blunt distal end in some embodiments, orit can include a pointed distal tip configured to puncture tissue tofacilitate introduction of the antenna 502 into a target volume oftissue.

The at least one fluid channel, such as the illustrated fluid channels504, 506, can be coupled to a fluid reservoir 508 by at least one fluidconduit 510. The fluid conduit 510 can be, for example, a length offlexible plastic tubing. The fluid conduit 510 can also be a rigid tube,or a combination of rigid and flexible tubing. The fluid reservoir 508can have a variety of geometries and sizes. In one embodiment, the fluidreservoir 508 can be a cylindrical container similar to a syringe barrelthat can be used with a linear pump, as described below.

Fluid can be urged from the fluid reservoir 508 into the fluid channels504, 506 of the antenna 502 by a pump 512. In one embodiment, the pump512 can be a syringe-type pump that produces a fixed volume flow vialinear advancement of a plunger (not shown). In other embodiments,however, other types of pumps, such as a diaphragm pump, may also beemployed.

The pump 512, as well as any other components of the system, can becontrolled by a controller 514. The controller 514 can include a powersupply and can be configured to deliver electrical control signals tothe pump 512 to cause the pump to produce a desired flow rate of fluid.The controller 514 can be connected to the pump 512 via an electricalconnection 516. The controller 514 can also be electrically coupled tothe antenna 502 using any known electrical connections of any desiredlength, though typically this connection is formed using a coaxial cablehaving an inner conductor and an outer conductor to mimic, for example,the structure of the antenna 200. Regardless of the physical form of theconnection, an inner emitting conductor 518 of the antenna 502 can beelectrically coupled to the power supply and controller 514 using anelectrical connection 520. Similarly, an outer return conductor 522 canbe coupled to the power supply and controller 514 using an electricalconnection 524. In addition, the controller 118 can be connected to anyheating assemblies disposed within the fluid channels 504, 506 (notshown) through a similar electrical connection.

In operation, the controller 514 can drive the delivery of fluid intotarget tissue at a desired flow rate, the heating of the fluid to adesired therapeutic temperature, and the delivery of microwave ablativeenergy via the antenna 502. To do so, the controller 514 can itselfcomprise a number of components for generating, regulating, anddelivering required electrical control and therapeutic energy signals.In addition to the power supply mentioned above, the controller 514 caninclude one or more digital data processors and associated storagememories that can be configured to perform a variety of functions, orcontrol discrete circuit elements that perform a given function. Thesefunctions can include, for example, the generation of one or moreelectrical signals of various frequencies and amplitudes. Furthermore,the controller 514 can be configured to amplify any of these signalsusing one or more power amplifiers. These amplified signals can bedelivered to the antenna 502 via one or more electrical connections 520,passed through tissue surrounding the antenna 502, and returned to thepower supply and controller 514 via the electrical connections 524. Thecontroller 514 can also include a number of other components, such as adirectional coupler to feed a portion of the one or more microwavesignals to, for example, a power monitor to permit adjustment of themicrowave signal power to a desired treatment level. Still further, thecontroller 514 can include a user interface to allow an operator tointeract with the controller and set desired therapy operatingparameters or receive feedback from the controller (e.g., warnings,indications, etc.).

As mentioned above, the triaxial antenna 200 is one of a variety ofmicrowave antenna architectures known in the art. FIG. 6 illustrates anembodiment of a device 600 configured for use with a fluid enhancedablation therapy system that utilizes a different embodiment of amicrowave antenna. The device 600 can include an elongate body 602having an inner lumen extending therethrough that is in communicationwith tissue surrounding the elongate body 602 via an opening 604 formedon a distal portion of the elongate body. An arced-shape microwaveantenna 606 can be housed within the inner lumen and it can beconfigured to axially translate to extend through the opening 604 intothe tissue surrounding the elongate body 602, as shown in the figure.The arced-shape antenna 606 can define a plane, i.e., it can trace aportion of a circle or other arc that lies within a plane (e.g., theplane of the page in FIG. 6). The arced-shape configuration of theantenna 606 can produce a roughly spherical microwave energy depositionfield. An exemplary arced-shape antenna was formerly sold under thetrade name VIVARING by Vivant/Valleylab.

Also housed within the inner lumen of the elongate body 602 can be afluid passageway 608 having an opening formed at or near a distal end609 thereof. Similar to the antenna 606, the fluid passageway 608 can behoused within the inner lumen of the elongate body 602 during insertioninto a volume of tissue, and it can be configured to axially translateto extend from the opening 604 such that the distal end 609 ispositioned at or near a center point of the arced-shape antenna 606, asshown in FIG. 6.

In use, fluid can be injected into the tissue surrounding the device 600from the opening formed near the distal end 609 of the fluid passageway608 at the same time that microwave energy is delivered into the tissuefrom the arc-shape antenna 606. The introduction of fluid at or near thecenter point of the arced-shape antenna is similar to the introductionof saline from the at least one opening 212 shown in FIG. 2. That is,saline or any other suitable fluid injected at this location acts as apoint source, and the saline flows radially in a spherical pattern fromthe distal end of the fluid passageway 608. Such a source of saline canhelp the therapy temperature profile from microwave energy depositiongrow in a spherical pattern.

The device 600 can be used alone, as shown in FIG. 6, or it can be usedin combination with one or more other devices to create a more sphericalvolume of therapeutically treated tissue. FIG. 7 illustrates oneembodiment of an assembly that uses multiple arced-shape antennas tocreate such a pattern. In particular, three elongate bodies 702, 704,706 can be positioned within a target volume of tissue such that theirarced-shape microwave antennas 708, 710, 712 define planes that areangularly offset from one another and that intersect one another. In theillustrated embodiment, for example, the planes defined by the threemicrowave antennas 708, 710, 712 are each offset from one another by120° such that they define three great circles (i.e., a circle on thesurface of a sphere that lies in a plane passing through the sphere'scenter) of the spherical volume of tissue disposed between the antennas.In other embodiments, however, the antennas can be angularly offset byother amounts, including, for example, 90° (i.e., perpendicularorientation). Regardless, a fourth elongate body 714 can be introducedparallel to the elongate bodies 702, 704, 706 to introduce fluid intothe target tissue volume. The elongate body 714 can be positionedvertically such that the fluid is introduced at or near the center ofthe sphere defined by the arced-shape microwave antennas 708, 710, 712.

FIG. 8 shows a top view of the elongate bodies 702, 704, 706, 714, alongwith the arced-shape antennas 708, 710, 712. From this view, the angularoffset of the planes defined by the antennas 708, 710, 712 and thecentral location of the elongate shaft 714 injecting fluid into thevolume of tissue are visible. In such an embodiment, the flow of fluidfrom the elongate body 714 can be perpendicular to the energy depositionpattern and can moderate the peak temperatures, make the temperature inthe therapy zone more uniform, and extend the heating field deeper intothe tissue than is possible by using the antennas 708, 710, 712 alone.

Another alternative embodiment of a microwave antenna for use in fluidenhanced ablation therapy is shown in FIG. 9. The antenna 900 caninclude an inner conductor 912, a dielectric insulator 914, an outerconductor 916, and a shield 918 in a similar arrangement as antenna 200.In this embodiment, the microwave antenna 900 can also include dualfluid channels 904, 906 symmetrically disposed on opposing sides of alongitudinal axis 908 of the antenna. In an alternative embodiment, asingle annular channel can be provided extending around thecircumference of the antenna 900. Each fluid channel 904, 906 caninclude a plurality of linearly arranged openings 910 extending alongthe longitudinal axis 908 of the microwave antenna 900. In such anembodiment, the point saline source of FIG. 2 is extended by includingthe longer array of openings 910. As a result, this design can injectsaline into the tissue as a line source, and the resultant saline flowwithin the target volume of tissue can be radially outward andcylindrically symmetric, rather than spherically symmetric as in theembodiment of FIG. 2. The resulting thermal field can be a substantiallyuniform temperature field extending cylindrically around the antenna900.

Regardless of the antenna architecture used, in some embodiments, highfluid flow rates can provide too much cooling to the therapy. Forexample, the saline flow rate needed to effectively carry the heatdelivered into the tissue can be significant, e.g., as high as 50 ml/minor more. Saline flow into the tissue at such rates can substantiallyquench the thermal therapy, keeping the tissue below a therapeutictemperature. To combat this problem, or to impart additional energy intothe tissue even at lower fluid flow rates, the saline or other suitablefluid can be heated to a therapeutic temperature prior to beingintroduced into the target volume of tissue.

FIG. 10 illustrates one embodiment of a microwave antenna 1000 that issimilar to the triaxial fluid enhanced ablation antenna 900 describedabove. It similarly includes an inner conductor 1020, dielectricinsulator 1022, outer conductor 1024, and shield 1026 arranged in thesame configuration. The antenna can also include at least one fluidchannel, such as dual fluid channels 1002, 1004. However, each of thefluid channels in this exemplary embodiment can further include at leastone heating element disposed therein. In the illustrated embodiment, forexample, the heating elements 1006, 1008 are disposed in the two fluidchannels 1002, 1004, respectively. The heating elements can be used toheat fluid flowing through the fluid channels 1002, 1004 prior to theirintroduction into tissue surrounding the antenna 1000 via a plurality ofopenings 1010 formed in each of the fluid channels.

The heating elements 1006, 1008 can have a variety of forms but, in someembodiments, can be dual-wire electrical heating assemblies that pass RFelectrical energy between two wires 1012 to heat the fluid. The wirescan be insulated along a proximal portion thereof extending from a powersource, as described above. The wires 1012 can be exposed along a distalportion thereof to allow the passage of energy therebetween, andelectrical shorts can be prevented by non-conducting spacers 1014 thathold the wires 1012 apart from one another. A thermocouple 1016, 1018 orother temperature monitoring element can also be disposed within thefluid channels 1002, 1004 to aid an operator or control system inregulating the power applied to the heating elements 1006, 1008 toachieve the desired temperature of fluid prior to introduction intotissue. Exemplary embodiments of the dual-wire heating elements 1006,1008 are described in U.S. Pat. Pub. No. 2012/0265190, which isincorporated by reference above.

In addition to the dual-wire heating elements described above,alternative forms of heating elements can also be suitable for use withthe devices and methods described herein. For example, instead of usingtwo separated wires, a heating element can utilize any two or more metalelectrodes in contact with the saline or other suitable fluid that canpass electrical current, thereby heating the saline resistively. Anotherembodiment described in U.S. Pat. Pub. No. 2012/0265190, incorporated byreference above, utilizes a single wire disposed within a fluid channelformed of an electrically conductive material. As a result, the fluidchannel itself acts as the second electrode in combination with thesingle wire disposed within the channel. Similar to the dual-wireheating element described above, a non-conducting spacer element can beutilized to prevent electrical shorts between the single wire and theconductive walls of the fluid channel.

In still other embodiments, a heating element can be a high-frequencymicrowave antenna whose frequency is selected such that the energy fieldit generates is fully dissipated within the fluid channel. Similarly, aheating element can employ an electrically resistive material in contactwith the saline that is heated via electrical power such that theresulting dissipated energy can be passed into the saline throughthermal conduction.

In certain microwave antenna architectures known in the art, a cryogenicgas has been used to control internal heating of a proximal portion of amicrowave antenna and the cabling connecting the antenna to a powersource. One such embodiment of a cryogenically cooled microwave ablationsystem is described in U.S. Pat. No. 7,244,254, which is herebyincorporated by reference in its entirety. As described in thatreference, heating of a cable and the proximal portion of a microwaveantenna coupled thereto can be controlled by the use of a cooled gas,ideally a cryogenic gas that can exceptionally cool when expanded due tothe Joule-Thompson effect. In use, a cryogenic gas is introduced into aninternal gas feed line disposed along the length of the power supplycable. The gas travels through the feed line toward the proximal end ofthe antenna and is allowed to expand as it reaches the antenna. Theexpansion of the gas near the proximal end of the antenna caneffectively cool that portion of the antenna, and the gas cansubsequently be routed to a vent along a proximal portion of the cable.

This design can provide adequate cooling of the cable and antenna, butdoes not remedy the challenges encountered with conventional microwaveablation related to treating larger volumes of tissue, as explainedabove. FIGS. 11-13 illustrate one embodiment of a cryogenically cooledmicrowave antenna assembly configured for use with fluid enhancedablation therapy.

With reference to FIG. 11, a microwave antenna assembly 1100 is shownthat includes a microwave power cable 1102 and an antenna 1104. Theillustrated antenna 1104 can be a monopole or dipole antenna, but theprinciples of the invention can be applied to other microwave antennaarchitectures as well. As shown in the figure, an outer return conductor1106 can extend along an outer surface of the antenna assembly 1100 fromthe antenna 1104 to the cable 1102 and ultimately to a power supply,such as the power supply and controller 514 discussed above. The outerreturn conductor 1106 can define an inner lumen 1107, and an inneremitting conductor 1108 can be disposed within the inner lumen 1107 in aproximal portion of the cable 1102. A similar inner emitting conductor1110 can be disposed in a distal portion of the antenna 1110. The innerconductors 1108, 1110 can be coupled by an inner gas feed line 1112 thatextends along the length of a distal portion of the cable 1102 and aproximal portion of the antenna 1104. The gas feed line 1112 can beformed from a metal tube so that it can effectively conduct microwavepower from inner emitting conductor 1108 to inner emitting conductor1110.

A proximal portion of the power cable 1102 can also include a solidstructural support 1114 disposed therein and filling the inner lumen1107. The outer return conductor 1106 and the structural support 1114can have a gas supply inlet 1116 extending therethrough that is in fluidcommunication with the gas feed line 1112. The gas supply inlet 1116 canbe, for example, a radially-oriented tube that passes through the outerreturn conductor 1106 and the solid structural support 1114.

Cryogenic gas that enters the gas supply inlet 1116 can be carriedforward toward the proximal end of the antenna 1104 within the gas feedline 1112. Upon reaching the proximal end of the antenna 1104, thecryogenic gas can be allowed to expand by passing through a gasexpansion conduit 1118 into an annular-shape gas return conduit 1120that is disposed around an outer circumference of the inner lumen 1107.The cryogenic gas can provide additional cooling as it expands into thelarger annular gas return conduit 1120 due to the Joule-Thompson effect.The expanded gas can then be carried back toward the proximal end of thecable 1102 to be expelled through a gas vent 1122.

In addition to the various gas supply lines, the inner lumen 1107 canalso house fluid supply lines configured to carry fluid from a sourceconnected to the proximal end of the cable 1102 to the antenna 1104. Forexample, in some embodiments a fluid supply line 1124 can extend from aproximal end of the cable 1102 through the structural support 1114.After passing through the structural support 1114, the fluid supply linecan, in some embodiments, expand to fill the annular conduit 1126 thatextends along the distal portion of the cable 1102 and the proximalportion of the antenna 1104 between the outer wall of the gas feed line1112 and the inner wall of the gas return conduit 1120. Once the flowingfluid reaches the distal end of the antenna 1104, it can be injectedinto tissue surrounding the antenna via at least one opening, e.g., aplurality of radially oriented openings 1128 formed in the antenna 1104(similar to the operation of the antenna 200 described above). Theintroduction of fluid into the tissue can aid in delivering thermalenergy deeper into the tissue than is possible with microwave ablationalone.

The fluid, typically saline, is in good thermal communication with thereturning cooled gas in the gas return conduit 1120. As a result, thefluid can be cooled as it travels along the length of the cable 1102 andantenna 1104. Indeed, in some embodiments, the fluid can be cooled tobelow body temperature prior to being injected into tissue. In the eventthat this is not desirable, one or more heating elements can be placedwithin the annular conduit 1126 to heat the fluid flowing therethrough.In the illustrated embodiment, for example, two or more RF electrodes1130 can be oriented along a longitudinal axis 1132 of the fluid annularconduit 1126 for fluid supply conduit 1124. The electrodes 1130 can bepassed through the structural support 1114 to reach a power supply at aproximal end of the cable 1102 and, if more than two are used, emittingelectrodes can be alternated with return electrodes, as shown by emitterelectrodes 1130A and return electrodes 1130B in FIGS. 12 and 13.

The one or more heating elements disposed within the inner lumen 1107can be used to heat the fluid flowing therethrough to any desiredtemperature. A temperature sensor (not shown) can be positioned in theantenna 1104 proximal to the openings 1128 to detect the temperature ofthe fluid being injected into the tissue surrounding the antenna 1104.This feedback can be used to control the amount of energy applied to theheating elements and achieve a desired temperature for the fluid uponinjection into tissue.

Furthermore, as a result of the fact that the fluid can be cooledsignificantly by the cryogenic gas, any desired temperature between thesolid phase change temperature and the gas phase change temperature(e.g., approximately 0° C. and 100° C., respectively, for saline) can beachieved by balancing the amount of cooling gas provided and the RFenergy applied to the one or more electrodes 1130. This provides a novelbenefit over prior art solutions, as it has not previously been possibleto introduce fluid into the tissue at a temperature below at least roomtemperature, if not body temperature.

FIG. 12 shows a cross-sectional view of the cable 1102 at location A-Ashown in FIG. 11. Visible in the figure are the outer return conductor1106, gas feed line 1112, solid structural support 1114, gas supplyinlet 1116, fluid supply line 1124, RF emitter electrodes 1130A, and RFreturn electrodes 1130B (extending in alternating pairs around thecircumference of the inner lumen 1107).

FIG. 13 similarly shows a cross-sectional view of the antenna 1104 atlocation B-B shown in FIG. 11. Visible in this figure are the outerreturn conductor 1106, gas feed line 1112, gas expansion conduit 1118,annular gas return conduit 1120, annular conduit 1126, RF emitterelectrodes 1130A, and RF return electrodes 1130B (again shown extendingin alternating pairs around the circumference of the inner lumen 1107).

Those skilled in the art will appreciate that a number of variations arepossible on the embodiment shown in FIGS. 11-13. For example, a numberof alternative heating elements can be employed in place of the RFelectrodes, such as laser-based heaters, resistive heaters, etc.Furthermore, there can be more than one heating element disposed alongthe length of the assembly 1100. For example, one or more proximalheating elements can be positioned near a proximal end of the cable 1102and used to control the temperature of a fluid prior to entering asecond, more distal set of one of more heating elements. This second setof heating elements can, for example, be positioned just proximal to theplurality of openings 1128 and used to control the temperature of thefluid prior to injection into tissue surrounding the antenna 1104. Inthis manner, the temperature of the microwave cable and antenna proximalto the therapy zone can be controlled by the first, proximal set ofheating elements to be at a sub-therapeutic temperature (e.g., typically41° C. or cooler to avoid destroying tissue) and a second, more distalset of heating elements can be used to further raise the temperature toa therapeutic level (e.g., above 41° C.).

Accordingly, methods according to the teachings of the present inventioncan include, for example, delivering therapeutic energy through amicrowave antenna configured to be positioned within a volume of tissue,while simultaneously delivering heated fluid into the volume of tissuethrough at least one outlet port formed in the microwave antenna. Incertain embodiments, the fluid can be heated solely by contacting themicrowave antenna and drawing excess heat therefrom. In otherembodiments, however, the excess heat from the microwave antenna may notheat the fluid to a desired therapeutic temperature. In such anembodiment, one or more heating elements disposed within the microwaveantenna (e.g., within a fluid passageway formed in the antenna) can beused to raise the temperature of the heated fluid prior to itsintroduction into tissue surrounding the antenna.

In other embodiments, a method for ablating tissue according to theteachings of the present invention can include delivering therapeuticenergy into a volume of tissue using a microwave antenna, and coolingthe microwave antenna by delivering a cryogenic gas through at least onegas channel formed in the microwave antenna. The method can furtherinclude delivering a fluid into the volume of tissue through at leastone fluid channel formed in the microwave antenna, and controlling thetemperature of the fluid delivered into the volume of tissue using thecooling of the cryogenic gas and one or more heating elements disposedwithin the at least one fluid channel. As mentioned above, by balancingthe cooling effects of the cryogenic gas with the output of the one ormore heating elements, the fluid can be controlled to enter the tissuesurrounding the antenna at any temperature between, e.g., a freezing anda boiling temperature of the fluid.

Any devices disclosed herein can be designed to be disposed after asingle use, or they can be designed for multiple uses. In either case,however, the devices can be reconditioned for reuse after at least oneuse. Reconditioning can include any combination of the steps ofdisassembly of a device, followed by cleaning or replacement ofparticular pieces, and subsequent reassembly. In particular, a devicecan be disassembled, and any number of the particular pieces or parts ofthe device can be selectively replaced or removed in any combination.Upon cleaning and/or replacement of particular parts, a device can bereassembled for subsequent use either at a reconditioning facility or bya surgical team immediately prior to a surgical procedure. Those skilledin the art will appreciate that the reconditioning of a device canutilize a variety of techniques for disassembly, cleaning/replacement,and reassembly. Use of such techniques, and the resulting reconditioneddevice, are all within the scope of the present invention.

Preferably, any devices described herein will be processed beforesurgery. First, a new or used instrument can be obtained and, ifnecessary, cleaned. The instrument can then be sterilized. In onesterilization technique, the instrument is placed in a closed and sealedcontainer, such as a plastic or TYVEK bag. The container and itscontents can then be placed in a field of radiation that can penetratethe container, such as gamma radiation, x-rays, or high-energyelectrons. The radiation can kill bacteria on the instrument and in thecontainer. The sterilized instrument can then be stored in the sterilecontainer. The sealed container can keep the instrument sterile until itis opened in the medical facility. Other sterilization techniques caninclude beta radiation, ethylene oxide, steam, and a liquid bath (e.g.,cold soak). In certain embodiments, the materials selected for use informing certain components may not be able to withstand certain forms ofsterilization, such as gamma radiation. In such a case, suitablealternative forms of sterilization can be used, such as ethylene oxide.

All papers and publications cited herein are hereby incorporated byreference in their entirety. One skilled in the art will appreciatefurther features and advantages of the invention based on theabove-described embodiments. Accordingly, the invention is not to belimited by what has been particularly shown and described, except asindicated by the appended claims.

What is claimed is:
 1. A microwave ablation device, comprising: anelongate body having an inner conducting element surrounded by adielectric insulator, an outer conducting element coaxially disposedaround the dielectric insulator and defining an outer wall, and aplurality of separate fluid channels extending through the dielectricinsulator parallel to a longitudinal axis of the microwave antenna, eachfluid channel including at least one opening positioned proximal to adistal end of the outer conducting element and extending through thedielectric insulator and the outer conducting element to allow fluid toflow into tissue surrounding the elongate body; wherein the inner andouter conducting elements are configured to deliver microwave energy totissue surrounding the elongate body, the dielectric insulator extendsdistally beyond the distal end of the outer conducting element, and eachfluid channel terminates at a location proximal to a distal end of thedielectric insulator.
 2. The device of claim 1, wherein the at least oneopening comprises a plurality of openings spaced along a longitudinalaxis of each of the plurality of fluid channels.
 3. The device of claim1, wherein the at least one opening comprises a plurality of openingsspaced circumferentially around a longitudinal axis of each of theplurality of fluid channels.
 4. The device of claim 1, wherein themicrowave antenna has a cylindrical shape, and wherein the innerconducting element, dielectric insulator, and outer conducting elementare coaxially aligned.
 5. The device of claim 1, wherein each of theplurality of fluid channels, includes a heating element disposedtherein, the heating element being positioned proximal to the at leastone opening and configured to heat fluid flowing through the channel. 6.The device of claim 1, further comprising a shield disposed around andcoaxially aligned with the outer conducting element.
 7. The device ofclaim 1, wherein the distal end of the dielectric insulator ispositioned at a distal end of the inner conductor.
 8. An ablationdevice, comprising: a microwave antenna having a hollow outer conductingelement, a coaxial inner conducting element extending through the outerconducting element, an annular fluid channel surrounding the innerconducting element, an inner lumen extending through the innerconducting element, an annular return channel in fluid communicationwith the inner lumen, isolated from the fluid channel, and positionedbetween the fluid channel and the outer conductor; and at least oneoutlet port formed in the outer conducting element at a distal end ofthe antenna and in fluid communication with the fluid channel, the atleast one outlet port being configured to deliver fluid flowing throughthe fluid channel into tissue surrounding the microwave antenna; whereinthe inner lumen is configured to receive a cryogenic gas and vent thecryogenic gas into the return channel such that the cryogenic gas doesnot enter the fluid channel.
 9. The device of claim 8, furthercomprising at least one heating element disposed in the fluid channeland configured to heat fluid flowing through the channel.
 10. The deviceof claim 8, wherein the at least one outlet port formed in the outerconducting element is positioned distally of a distal end of the returnchannel.